1. Field of the Invention
This invention relates to substituted 3,3-diphenylphthalides useful as color precursors, substituted 2-benzoylbenzoic acids useful as intermediates in preparing them, processes for preparing them, and pressure-sensitive duplicating systems and heat-sensitive marking systems comprising them.
2. Description of the Prior Art
In the art of pressure-sensitive duplicating systems and heat-sensitive marking systems color precursors in current use are colorless under neutral or basic conditions, but become colored when contacted with an acidic material such as silica gel, a phenolic resin or an acidic clay. Two known diphenylphthalide color precursors are malachite green lactone of Formula A, which is described in Beilsteins Handbuch der Organischen Chemie (Vierte Auflage, Band XVIII, Seite 617, 1934), and crystal violet lactone of Formula B, which is described in U.S. Pat. No. 2,417,897 (Reissue 23,024). ##STR1##
Two of many patents which describe pressure-sensitive duplicating systems containing malachite green lactone and crystal violet lactone are U.S. Pat. No. 2,548,364 and U.S. Pat. No. 2,548,365. In such systems malachite green lactone and crystal violet lactone, the latter of which is widely used, produce images whose colors are intense but not deep. Depth of color is desired for easier readability and better xerographic copiability of the images. In order to overcome lack of color depth, other color precursors have been mixed with crystal violet lactone as described, for example, in U.S. Pat. No. 3,525,630. An unsolved problem in the art has been both intensity and depth of color in the same molecule. The present invention is a solution to the problem.